This document provides information about membrane filtration technologies used for water treatment. It discusses different types of membrane filtration processes like microfiltration, ultrafiltration, reverse osmosis, and nanofiltration. It also describes common membrane materials, module designs, and operating modes like deposition and suspension. The document contains detailed technical descriptions of membrane components and configurations.

1. ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬
‫السعودية‬ ‫العربية‬ ‫المملكة‬
‫التعليم‬ ‫وزارة‬
‫الباحة‬ ‫جامعة‬
‫والبيئة‬ ‫المياه‬ ‫في‬ ‫مختارة‬ ‫موضوعات‬
Membrane Filtration
High-pressure membranes

2. 2
table of Contents
Page
Membrane Filtration
3
MEMBRANE MODULES 4
MEMBRANE MATERIALS 6
High-pressure membranes
7
Reverse osmosis (RO) 7
Nanofiltration (NF) 13
New developments 18
A model for water treatment at the Saudi
Desalination Plant
19

3. 3
Membrane Filtration
Membranes are thin and porous sheets of material able to separate contaminants
from water when a driving force is applied. Once considered a viable technology only
for desalination, membrane processes are increasingly employed in both drinking
water and wastewater treatment for removal of bacteria and other microorganisms,
particulate material, micropollutants, and natural organic material, which can impart
colour, tastes, and odours to the water and react with disinfectants to form
disinfection by-products (DBP).
As advancements are made in membrane production and module design, capital
and operating costs continue to decline.
Water treatment processes employ several types of membranes. They include
microfiltration (M-F), ultrafiltration (U-F), reverse osmosis (R-O), and nanofiltration
(N-F) membranes. Microfiltration membranes have the largest pore size and typically
reject large particles and various microorganisms.
Ultrafiltration membranes have smaller pores than microfiltration membranes and
therefore, in addition to large particles and microorganisms, they can reject bacteria
and soluble macromolecules such as proteins. Reverse osmosis membranes are
effectively non-porous and, therefore, exclude particles and even many low molar
mass species such as salt ions, organics, etc. Nanofiltration membranes are relatively
new and are sometimes called “loose” reverse osmosis membranes.
They are porous membranes, but since the pores are on the order of ten angstroms
or less, they exhibit performance between that of reverse osmosis and ultrafiltration
membranes.

4. 4
MEMBRANEMODULES
Membrane filters are usually manufactured as flat sheet stock or as hollow fibers
and then formed into on of several different types of membrane modules. Module
construction typically involves potting or sealing the membrane material into an
assembly, such as with hollow-fiber module. These types of modules are designed
for long-term use over the course of a number of years. Spiral-wound modules are
also manufactured for long-term use, although these modules are encased in a
separate pressure vessel that is independent of the module itself. Hollow-Fiber
Modules Most hallow-fiber modules used in drinking water treatment applications
are manufactured for MF or UF membranes to filter particulate matter. These
modules are comprised of hollow-fiber membranes, which are long and very narrow
tubes that may be constructed of membrane materials described previously. The
fibers may be bundled in one of several different arrangements. Fibers can be
bundled together longitudinally, potted in a resin on both ends, and encased in a
pressure vessel. These modules are typically mounted vertically, although horizontal
mounting may be used. These fibers can be similar to spiral-wound modules and
inserted into pressure vessels independent of the module itself. These modules (and
the pressure vessels) are mounted horizontally. Bundled hollow fibers can also be
vertically and submerged in a basin that does not need a pressure vessel. • Fiber wall
thickness 0.1-0.6 mm • Fiber length 1-2 meters Hollow-fiber membrane modules
may operate in an “inside-out” or “outside-in” mode. In insideout mode, feed water
enters the center of the fiber (lumen) and is filtered radially through the fiber wall.
Filtrate is then collected from outside the fiber. During outside-in operation, feed
water passes fromoutside the fiber to the inside, where filtrate is collected in the
center of the fiber. Hollow Fiber Cross-Section Photomicrograph When a hollow-
fiber module is operated in an inside-out mode, pressurized feed water may enter
the center of the fiber at either end of the module, while filtrate exits through a port
located at the center or end of the module. In outside-in mode, feed water typically
enters the module through an inlet port located in the center and is filtered into the
center of the fiber, where the filtrate exits through a port at one end of the module.
Most hollow-fiber systems operate in direct filtration mode and are periodically
backwashed to remove the accumulated solids. Membrane Filtration Spiral-Wound
Modules Spiral-wound modules were developed to remove dissolved solids, and are
most often associated with NF/RO processes. The basic unit is a sandwich of flat
membrane sheets called a “leaf” wound around a central perforated tube. One leaf
consists of two membrane sheets placed back to back and separated by a spacer
called permeate carrier. Layers of the leaf are glued along three edges, while the
unglued edge is sealed around the perforated central tube. Feed water enters the

5. 5
spacer channels at the end of the spiral-wound element in a path parallel to the
central tube. As feed water flows through the spacers, a portion permeates through
either of the two surrounding membrane layers and into the permeate carrier,
leaving behind any dissolved and particulate contaminates that are rejected by the
membrane. Filtered water in the permeate carrier travels spirally inward toward the
central collector tube, while water in the feed spacer that does not permeate
through the membrane continues to flow across the membrane surface, becoming
increasingly concentrated with rejected contaminates. This concentrate stream exits
the element parallel to the central tube through the opposite end from which the
feed water entered. Inside-Out and Outside-In Modes of Operation (Using Pressure
Vessels) Membrane Filtration Spiral-Wound Membrane Module DEPOSITION MODE
Membrane filtration systems operating in deposition have one influent (feed) and
one effluent (filtrate) stream. These systems are also commonly called “dead-end” or
“direct” filtration systems and are similar to conventional granular media filters in
terms of hydraulic configuration. In deposition mode, contaminates suspended in
the feed stream accumulate on the membrane surface and are held in place by
hydraulic forces acting perpendicular to the membrane, forming a cake layer.
Schematic of a System Operating in Deposition Mode Membrane Filtration
Deposition Mode Most hollow-fiber MF and UF systems operate in deposition mode.
Typically, accumulated solids are removed from MF/UF systems by backwashing.
However, some systems operate until accumulated solids reduce the flow and/or
TMP to an unacceptable level, at which point the membrane cartridge is replaced.
Some MF/UF systems utilize a periodic “backpulse” or a short interval of reverse
flow (which may include air and/or addition of small doses of oxidants) designed to
dislodge particles from the membrane surface without removing these solids from
the system. This process re-suspends particles, effectively concentrating the
suspended solids in the feed near the membrane surface and increasing the
potential for pathogens or other particulate to pass through an integrity breach and
contaminate the filtrate. SUSPENSION MODE In membrane filtration systems that
operate in suspension mode, a scouring force using water and/or air is applied
parallel to the membrane during production of the filtrate in a continuous or
intermittent manner. The objective of operating in this mode is to minimize the
accumulation of contaminants at the membrane surface or boundary layer, thus
reducing fouling.

6. 6
MEMBRANEMATERIALS
Normally, membrane material is manufactured from a synthetic polymer, although
other forms, including ceramic and metallic “membranes,” may be available.
Almost all membranes manufactured for drinking water are made of polymeric
material, since they are significantly less expensive than membranes constructed of
other materials. Membranes constructed of polymers that react with oxidants used
in drinking water treatment should not be used with chlorinated feed water.
Mechanical strength is another consideration, since a membrane with greater
strength can withstand larger trans-membrane pressure (TMP) levels, allowing for
greater operational flexibility and the use of higher pressures.
Membranes with bi-directional strength may allow cleaning operations or integrity
testing to be performed from either feed or filtrate side of the membrane.
Membranes with a particular surface charge may remove particulate or microbial
contaminants of the opposite charge due to Membrane Filtration 4 electrostatic
attraction. Membranes can also be hydrophilic (water attracting) or hydrophobic
(water repelling). These terms describe how easily membranes can be wetted, as
well as its ability to resist fouling to some degree. MF and UF membranes may be
constructed from a wide variety of materials, including cellulose acetate,
polyvinylidene fluoride, polyacrylonitrile, polypropylene, polysulfone,
polyethersulfone, or other polymers. Each of these materials has different properties
with respect to the surface charge, degree of hydrophobicity, pH and oxidant
tolerance, strength and flexibility. NF and RO membranes are generally
manufactured from cellulose acetate or ployamide materials, and their various
advantages and disadvantages. Cellulose membranes are susceptible to
biodegradation and must be operated within a narrow, pH range of 4 to 8 but they
do have some resistance to continuous low-level oxidants. Chlorine doses of 0.5
mg/L or less may control biodegration and biological fouling without damaging the
membrane. Polyamide membranes, by contrast, can be used under a wide range of
pH conditions and are not subject to biodegradation. Although these membranes
have very limited tolerance for strong oxidants, they are compatible with weaker
oxidants such as cholramines. These membranes require significantly less pressure
to operate and have become the predominate material used for NF or RO
applications.

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High-pressure membranes
 Reverse osmosis(RO)
 Nanofiltration(NF)
Reverse osmosis (RO) is a water purification technology that uses
a semipermeable membrane to remove ions, molecules, and larger particles
from drinking water.
In reverse osmosis, an applied pressure is used to overcome osmotic pressure,
a colligative property, that is driven by chemical potential differences of the
solvent, a thermodynamic parameter. Reverse osmosis can remove many types
of dissolved and suspended species from water, including bacteria, and is used in
both industrial processes and the production of potable water. The result is that
the solute is retained on the pressurized side of the membrane and the
pure solvent is allowed to pass to the other side. To be "selective", this
membrane should not allow large molecules or ions through the pores (holes),
but should allow smaller components of the solution (such as solvent molecules)
to pass freely.
In the normal osmosis process, the solvent naturally moves from an area of low
solute concentration (high water potential), through a membrane, to an area of
high solute concentration (low water potential).
The driving force for the movement of the solvent is the reduction in the free
energy of the systemwhen the difference in solvent concentration on either side
of a membrane is reduced, generating osmotic pressure due to the solvent
moving into the more concentrated solution. Applying an external pressure to
reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is
similar to other membrane technology applications. However, key differences
are found between reverse osmosis and filtration. The predominant removal
mechanism in membrane filtration is straining, or size exclusion, so the process
can theoretically achieve perfect efficiency regardless of parameters such as the
solution's pressure and concentration. Reverse osmosis also involves diffusion,
making the process dependent on pressure, flow rate, and other
conditions.[2] Reverse osmosis is most commonly known for its use in
drinking water purification from seawater, removing the salt and
other effluent materials from the water molecules.

8. 8
History
The process of osmosis through semipermeable membranes was first observed in
1748 by Jean-Antoine Nollet. For the following 200 years, osmosis was only a
phenomenon observed in the laboratory. In 1950, the University of California at Los
Angeles first investigated desalination of seawater using semipermeable
membranes. Researchers from both University of California at Los Angeles and
the University of Florida successfully produced fresh water from seawater in the
mid-1950s, but the flux was too low to be commercially viable until the discovery at
University of California at Los Angeles by Sidney Loeb and Srinivasa Sourirajan at
the National Research Council of Canada, Ottawa, of techniques for making
asymmetric membranes characterized by an effectively thin "skin" layer supported
atop a highly porous and much thicker substrate region of the membrane.
John Cadotte, of FilmTec Corporation, discovered that membranes with particularly
high flux and low salt passage could be made by interfacial polymerization
of m-phenylene diamine and trimesoyl chloride. Cadotte's patent on this
process was the subject of litigation and has since expired. Almost all commercial
reverse osmosis membrane is now made by this method. By the end of 2001, about
15,200 desalination plants were in operation or in the planning stages, worldwide,
with approximately 20 percent of them in the U.S., the largest number of any
country in the world. In terms of capacity however, the U.S. ranks second globally
Reverse osmosis production train, North Cape Coral Reverse Osmosis Plant
In 1977 Cape Coral, Florida became the first municipality in the United States to use
the RO process on a large scale with an initial operating capacity of 3 million gallons
(11350 m³) per day. By 1985, due to the rapid growth in population of Cape Coral,
the city had the largest low pressure reverse osmosis plant in the world, capable of
producing 15 million gallons per day (MGD) (56800 m³/d).

9. 9
Process
Osmosis is a natural process. When two solutions with different concentrations of a
solute are separated by a semipermeable membrane, the solvent has a tendency to
move from low to high solute concentrations for chemical potential equilibration.
Formally, reverse osmosis is the process of forcing a solvent from a region of high
solute concentration through a semipermeable membrane to a region of low solute
concentration by applying a pressure in excess of the osmotic pressure. The largest
and most important application of reverse osmosis is the separation of pure water
from seawater and brackish waters; seawater or brackish water is pressurized
against one surface of the membrane, causing transport of salt-depleted water
across the membrane and emergence of potable drinking water from the low-
pressure side.
The membranes used for reverse osmosis have a
dense layer in the polymer matrix—either the
skin of an asymmetric membrane or an
interfacially
polymerized layer within a thin-film-composite
membrane—where the separation occurs. In
most cases, the membrane is designed to allow
only water to pass through this dense layer, while
preventing the passage of solutes (such as salt
ions). This process requires that a high pressure
be exerted on the high concentration side of the
membrane, usually 2–17 bar (30–250 psi) for fresh and
brackish water, and 40–82 bar (600–1200 psi) for
seawater, which has around 27 bar (390 psi
natural osmotic pressure that must be overcome. This process is best known for its
use in desalination (removing the salt and other minerals from sea water to get fresh
water), but since the early 1970s, it has also been used to purify fresh water for
medical, industrial, and domestic application.
The typical single-pass seawater reverse osmosissystemconsists of:
 Intake
 Pretreatment
 High pressure pump (if not combined with energy recovery(
 Membrane assembly
 Energy recovery (if used(
 Remineralisation and pH adjustment
 Disinfection
 Alarm/control panelrm/control panel
A semipermeable membrane coil used in
desalination

10. 10
Pretreatment
Pretreatment is important when working with reverse osmosis and nanofiltration
membranes due to the nature of their spiral-wound design. The material is
engineered in such a fashion as to allow only one-way flow through the system. As
such, the spiral-wound design does not allow for backpulsing with water or air
agitation to scour its surface and remove solids. Since accumulated material cannot
be removed from the membrane surface systems, they are highly susceptible to
fouling (loss of production capacity). Therefore, pretreatment is a necessity for any
reverse osmosis or nanofiltration system. Pretreatment in sea water reverse osmosis
systems has four major components
Screening of solids: Solids within the water must be removed and the water treated
to prevent fouling of the membranes by fine particle or biological growth, and
reduce the risk of damage to high-pressure pump components
Cartridge filtration: Generally, string-wound polypropylene filters are used to
remove particles of 1–5 µm diameter
Dosing: Oxidizing biocides, such as chlorine, are added to kill bacteria, followed by
bisulfite dosing to deactivate the chlorine, which can destroy a thin-film composite
membrane. There are also biofouling inhibitors, which do not kill bacteria, but simply
prevent them from growing slime on the membrane surface and plant walls.
Prefiltration pH adjustment: If the pH, hardness and the alkalinity in the feedwater
result in a scaling tendency when they are concentrated in the reject stream, acid is
dosed to maintain carbonates in their soluble carbonic acid form Carbonic acid
cannot combine with calcium to form calcium carbonate scale. Calciumcarbonate
scaling tendency is estimated using the Langelier saturation index. Adding too much
sulfuric acid to control carbonate scales may result in calcium sulfate, barium sulfate,
or strontium sulfate scale formation on the reverse osmosis membrane
Prefiltration antiscalants: Scale inhibitors (also known as antiscalants) prevent
formation of all scales compared to acid, which can only prevent formation of
calciumcarbonate and calcium phosphate scales. In addition to inhibiting carbonate
and phosphate scales, antiscalants inhibit sulfate and fluoride scales and disperse
colloids and metal oxides. Despite claims that antiscalants can inhibit silica
formation, no concrete evidence proves that silica polymerization can be inhibited
by antiscalants. Antiscalants can control acid-soluble scales at a fraction of the
dosage required to control the same scale using sulfuric acid
Some small scale desalination units use 'beach wells'; they are usually drilled on the
seashore in close vicinity to the ocean.

11. 11
These intake facilities are relatively simple to build and the seawater they collect is
pretreated via slow filtration through the subsurface sand/seabed formations in the
area of source water extraction
Raw seawater collected using beach wells is often of better quality in terms of solids,
silt, oil and grease, natural organic contamination and aquatic microorganisms,
compared to open seawater intakes. Sometimes, beach intakes may also yield
source water of lower salinity
Highpressure pump
The high pressure pump supplies the pressure needed to push water through the
membrane, even as the membrane rejects the passage of salt through it. Typical
pressures for brackish water range from (15.5 to 26 bar, or 1.6 to 2.6 MPa).
In the case of seawater, they range from (55 to 81.5 bar or 6 to 8 MPa). This
requires a large amount of energy. Where energy recovery is used, part of the high
pressure pump's work is done by the energy recovery device, reducing the system
energy inputs.
Membrane assembly
The membrane assembly consists of a pressure vessel with a membrane that allows
feed water to be pressed against it. The membrane must be strong enough to
withstand whatever pressure is applied against it. Reverse osmosis membranes are
made in a variety of configurations, with the two most common configurations being
spiral-wound and hollow-fiber.
Only a part of the saline feed water pumped into the membrane assembly passes
through the membrane with the salt removed. The
remaining "concentrate" flow passes along the saline
side of the membrane to flush away the concentrated
salt solution. The percentage of desalinated water
produced versus the saline water feed flow is known as
the "recovery ratio". This varies with the salinity of the
feed water and the systemdesign parameters: typically
20% for small seawater systems, 40% – 50% for larger
seawater systems, and 80% – 85% for brackish water.
The concentrate flow is at typically only 3 bar / 50 psi
less than the feed pressure, and thus still carries much
of the high pressure pump input energy.
The desalinated water purity is a function of the feed
water salinity, membrane selection and recovery ratio.
To achieve higher purity a second pass can be added which generally requires re-
pumping. Purity expressed as total dissolved solids typically varies from 100 to 400
parts per million (ppm or milligram/litre)on a seawater feed. A level of 500 ppm is
The layers of a membrane

12. 12
generally accepted as the upper limit for drinking water, while the US Food and Drug
Administration classifies mineral water as water containing at least 250 ppm.
Disinfection
Post-treatment consists of preparing the water for distribution after filtration.
Reverse osmosis is an effective barrier to pathogens, but post-treatment provides
secondary protection against compromised membranes and downstream problems.
Disinfection by means of ultra violet (UV) lamps (sometimes called germicidal or
bactericidal) may be employed to sterilize pathogens which bypassed the reverse
osmosis process. Chlorination or chloramination (chlorine and ammonia) protects
against pathogens which may have lodged in the distribution system downstream,
such as from new construction, backwash, compromised pipes, etc.
Waste streamconsiderations
Depending upon the desired product, either the solvent or solute stream of reverse
osmosis will be waste. For food concentration applications, the concentrated solute
stream is the product and the solvent stream is waste. For water treatment
applications, the solvent stream is purified water and the solute stream is
concentrated waste.[30] The solvent waste stream from food processing may be
used as reclaimed water, but there may be fewer options for disposal of a
concentrated waste solute stream. Ships may use marine dumping and coastal
desalination plants typically use marine outfalls.

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Nanofiltration(NF)
Nanofiltration (NF) is a relatively recent membrane filtration process used most
often with low total dissolved solids water such as surface water and fresh
groundwater, with the purpose of softening (polyvalent cation removal) and removal
of disinfection by-product precursors such as natural organic matter and synthetic
organic matter.
Overview
Nanofiltration is a membrane filtration-based method that uses nanometer sized
cylindrical through-pores that pass through the membrane at 90°. Nanofiltration
membranes have pore sizes from 1-10 nanometers, smaller than that used in
microfiltration and ultrafiltration, but just larger than that in reverse osmosis.
Membranes used are predominantly created from polymer thin films. Materials that
are commonly used include polyethylene terephthalate or metals such as aluminum.
Pore dimensions are controlled by pH, temperature and time during development
with pore densities ranging from 1 to 106 pores per cm2. Membranes made from
polyethylene terephthalate and other similar materials, are referred to as “track-
etch” membranes, named after the way the pores on the membranes are made.
“Tracking” involves bombarding the polymer thin filmwith high energy particles.
This results in making tracks that are chemically developed into the membrane, or
“etched” into the membrane, which are the pores. Membranes created from metal
such as alumina membranes, are made by electrochemically growing a thin layer of
aluminum oxide from aluminum metal in an acidic medium.
Advantages and disadvantages
One of the main advantages of nanofiltration as a method of softening water is that
during the process of retaining calcium and magnesium ions while passing smaller
hydrated monovalent ions, filtration is performed without adding extra sodium ions,
as used in ion exchangers. Many separation processes do not operate at room
temperature (e.g. distillation), which greatly increases the cost of the process when
continuous heating or cooling is applied. Performing gentle molecular separation is
linked with nanofiltration that is often not included with other forms of separation
processes (centrifugation). These are two of the main benefits that are associated

14. 14
with nanofiltration. Nanofiltration has a very favorable benefit of being able to
process large volumes and continuously produce streams of products. Still,
Nanofiltration is the least used method of membrane filtration in industry as the
membrane pores sizes are limited to only a few nanometers. Anything smaller,
reverse osmosis is used and anything larger is used for ultrafiltration. Ultrafiltration
can also be used in cases where nanofiltration can be used, due to it being more
conventional. A main disadvantage associated with nanotechnology, as with all
membrane filter technology, is the cost and maintenance of the membranes used.[9]
Nanofiltration membranes are an expensive part of the process. Repairs and
replacement of membranes is dependent on total dissolved solids, flow rate and
components of the feed. With nanofiltration being used across various industries,
Designand operation
Industrial applications of membranes require hundreds to thousands of square
meters of membranes and therefore an efficient way to reduce the footprint by
packing them is required. Membranes first became commercially viable when low
cost methods of housing in ‘modules’ were achieved. Membranes are not self-
supporting. They need to be stayed by a porous support that can withstand the
pressures required to operate the NF membrane without hindering the performance
of the membrane. To do this effectively, the module needs to provide a channel to
remove the membrane permeation and provide appropriate flow condition that
reduces the phenomena of concentration polarisation. A good design minimises
pressure losses on both the feed side and permeate side and thus energy
requirements. Leakage of the feed into the permeate stream must also be
prevented. This can be done through either the use of permanent seals such as glue
or replaceable seals such as O-rings.
Concentration polarisation
Concentration polarisation describes the accumulation of the species being retained
close to the surface of the membrane which reduces separation capabilities.
It occurs because the particles are convected towards the membrane with the
solvent and its magnitude is the balance between this convection caused by solvent
flux and the particle transport away from the membrane due to the concentration
gradient (predominantly caused by diffusion.) Although concentration polarisation is
easily reversible, it can lead to fouling of the membrane.

15. 15
Spiral wound module
Spiral wound modules are the most commonly used style of module and are
‘standardized’ design,
available in a range of standard diameters (2.5”, 4” and 8”) to fit standard pressure
vessel that can hold several modules in series connected by O-rings. The module
uses flat sheets wrapped around a central tube. The membranes are glued along
three edges over a permeate spacer to form ‘leaves’. The permeate spacer supports
the membrane and conducts the permeate to the central permeate tube. Between
each leaf, a mesh like feed spacer is inserted.
The reason for the mesh like dimension of the spacer is to provide a hydrodynamic
environment near the surface of the membrane that discourages concentration
polarisation. Once the leaves have been wound around the central tube, the module
is wrapped in a casing layer and caps placed on the end of the cylinder to prevent
‘telescoping’ that can occur in high flow rate and pressure conditions.
Tubular module
Tubular modules look similar to shell and tube heat exchangers with bundles of
tubes with the active surface of the membrane on the inside. Flow through the tubes
is normally turbulent, ensuring low concentration polarisation but also increasing
energy costs. The tubes can either be self-supporting or supported by insertion into
perforated metal tubes. This module design is limited for nanofiltration by the
pressure they can withstand before bursting, limiting the maximum flux possible.
Due to both the high energy operating costs of turbulent flow and the limiting burst
pressure, tubular modules are more suited to ‘dirty’ applications where feeds have
particulates such as filtering raw water to gain potable water in the Fyne process.
The membranes can be easily cleaned through a ‘pigging’ technique with foam balls
are squeezed through the tubes, scouring the caked deposits.
Flux enhancing strategies
These strategies work to reduce the magnitude of concentration polarisation and
fouling. There is a range of techniques available however the most common is feed
channel spacers as described in spiral wound modules.
All of the strategies work by increasing eddies and generating a high shear in the
flow near the membrane surface. Some of these strategies include vibrating the
membrane, rotating the membrane, having a rotor disk above the membrane,
pulsing the feed flow rate and introducing gas bubbling close to the surface of the
membrane.

16. 16
Typical figures for industrial applications
Keeping in mind that NF is usually part of a composite system for purification, a
single unit is chosen based off the design specifications for the NF unit.
For drinking water purification many commercial membranes exist, coming from
different chemical families, having different structures, chemical tolerances and salt
rejections and so the characterisation must be chosen based on the chemical
composition and concentration of the feed stream.
NF units in drinking water purification range from extremely low salt rejection
(<5% in 1001A membranes) to almost complete rejection (99% in 8040-TS80-TSA
membranes.) Flow rates range from 25–60 m3/day for each unit, so commercial
filtration requires multiple NF units in parallel to process large quantities of feed
water. The pressures required in these units are generally between 4.5-7.5 bar.]
For seawater desalination using a NF-RO system a typical process is shown below.
Because of the fact that NF permeate is rarely clean enough to be used as the final
product for drinking water and other water purification, is it commonly used as a pre
treatment step for reverse osmosis (RO) as is shown above.

17. 17
Post-treatment
As with other membrane based separations such as ultrafiltration, microfiltration
and reverse osmosis, post-treatment of eitherpermeate or retentate flow streams
(depending on the application) – is a necessary stage in industrial NF separation prior
to commercial distribution of the product.
The choice and order of unit operations employed in post-treatment is dependent
on water quality regulations and the design of the NF system. Typical NF water
purification post-treatment stages include aeration and disinfection & stabilisation.
Aeration
A Polyvinyl chloride (PVC) or fibre-reinforced plastic (FRP) degasifier is used to
remove dissolved gases such as carbon dioxide and hydrogen sulfide from the
permeate stream.[15] This is achieved by blowing air in a countercurrent direction to
the water falling through packing material in the degasifier. The air effectively strips
the unwanted gases from the water.
Disinfectionandstabilisation
The permeate water from a NF separation is demineralised and may be disposed to
large changes in pH, thus providing a substantial risk of corrosion in piping and other
equipment components.
To increase the stability of the water, chemical addition of alkaline solutions such as
lime and caustic soda is employed. Furthermore, disinfectants such as chlorine or
chloroamine are added to the permeate, as well as phosphate or fluoride corrosion
inhibitors in some cases.

18. 18
New developments
Contemporary research in the area of Nanofiltration (NF) technology is primarily
concerned with improving the performance of NF membranes, minimising
membrane fouling and reducing energy requirements of already existing processes.
One way in which researchers are attempting to improve NF performance – more
specifically increase permeate flux and lower membrane resistance – is through
experimentation with different membrane materials and configurations.
thin film composite membranes (TFC), which consist of a number of extremely thin
selective layers interfacially polymerized over a microporous substrate, have had the
most commercial success in industrial membrane applications due to the capability
of optimizing the selectivity and permeability of each individual layer.Recent
research has shown that the addition of nanotechnology materials such as
electrospunnanofibrous membrane layers (ENMs) to conventional TFC membranes
results in an enhanced permeate flux.
This has been attributed to inherent properties of ENMs that favour flux, namely
their interconnected pore structure, high porosity and low transmembrane pressure.
A recently developed membrane configuration which offers a more energy efficient
alternative to the commonly used spiral wound arrangement is the hollow fibre
membrane. This format has the advantage of requiring significantly less pre-
treatment than spiral wound membranes, as solids introduced in the feed are
displaced effectively during backwash or flushing. As a result, membrane fouling and
pre-treatment energy costs are reduced. Extensive research has also been conducted
on the potential use of Titanium Dioxide (TiO2, titania) nanoparticles for membrane
fouling reduction. This method involves applying a nonporous coating of titania onto
the membrane surface. Internal fouling/pore blockage of the membrane is resisted
due to the nonporosity of the coating, whilst the superhydrophilic nature of titania
provides resistance to surface fouling by reducing adhesion of emulsified oil on the
membrane surface.

19. 19
A model for water treatment at the Saudi DesalinationPlant
Filtrationof seawaterinSaudi Arabia'sEasternRegionisacomplex processdue tohigh
salinity,the presence of oilsandgreases,seashallowness,seasonalredtides,andjellyfish.
The Al Khafji SWROdesalinationplantwill use ultrafiltration(UF) duringpre-treatmentand
RO to remove dissolvedimpuritiesand saltsfromthe Gulf .
Al Khafji SolarSWRODesalination
PlantLargestin WorldThe Al Khafji
SolarSaline WaterReverse Osmosis
(SolarSWRO) DesalinationPlant
nearthe KuwaitborderinSaudi
Arabia,due to be completedin
2017, issetto be the world’s
largestfacilityof itskind.
The SWRO processforcesseawaterthrougha polymermembrane usingpressuretoremove
salt.The seawaterwill passthroughanintake tower,whereaJellyfishfilterisinstalled.
A 6km pipelinewillcarrythe raw waterto the new solar-powereddesalinationplantfor
furthertreatment.
A dissolvedairflotation(DAF)systemwill remove oil andother solidsfromthe raw water.
Thisprocessincludesmixingairwithwaterunderpressure andreleasingatatmospheric
pressure ina floatationtankorbasin,whichresultsinthe formationof tinybubbles.The
suspendedmatterclingstothe airbubblesandfloatsonthe surface of the waterfrom
where itwill be removedusingaskimmingdevice.
The UF employedinthe secondstage willalsoremove marine organismsandother
contaminantsfromthe water.
will supply60,000m³ of desalinatedseawateradayto the city of Al Khafji innorth-eastern
Saudi Arabia,providingaregularsupplyof watertothe regionthroughoutthe year".
The desalinationplantwill includeadissolvedairflotationunit(DAF)system,asecond-stage
ultra-filtrationunit(UF),chemical-dosingsystems,areverse-osmosis(RO) unit,andstorage
tanksfor carbon dioxide (CO²),chlorine andlime,aswell asa centralisedcontrol roomfor
powerandwater.
The RO unitwill be dividedintosix ROtrains,whichwillallow optimumusage of variable
solarpowerlevels.
The membraneswill be highlyresistanttochlorine,saltblockage,andaccumulationof
bacteriainthe RO membrane.The plantwill alsofeature asystemtooptimise power
Al Khafji SWRO

20. 20
consumptionanda pre-treatmentphase toreduce the highlevelsof salinity,oils,andfatsin
the region'sseawater.
All motorswill be controlledwithvariablefrequencydrives(VFD) toallow high-variabilityof
operational capacity.